Photoion Efficiency Spectra, Ionization Energies - American Chemical

Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771. Received: December 9, 1993; In Final Form: Febru...
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J. Phys. Chem. 1994, 98, 4026-4033

4026

Photoionization of Hydroxymethyl (CD20H and CD20D) and Methoxy (CDJO) Radicals: Photoion Efficiency Spectra, Ionization Energies, and Thermochemistry Szu-Cberng Kuo, Zhengyu Zhang, and R. Bruce Klemm' Brookhaven National LaboratorylBldg. 81 5. P.O. Box 5000, Upton, New York 1 1 973-5000

Joel F. Liebman Department of Chemistry and Biochemistry, University of Maryland, Baltimore County Campus, Baltimore, Maryland 21228-5398

Louis J. Stief and Fred L. Nesbittt Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771 Received: December 9, 1993; In Final Form: February 7 , 1994@

Photoion efficiency (PIE) spectra were obtained for CDzOH, CDzOD, and CD3O radicals using the discharge flow-photoionization mass spectrometry technique. The radicals were generated in a flow tube via reaction of F atoms with the appropriate methanol isotopomers (CD30H CDzOH, CD3OD CDzOD, and C D 3 0 H CD3O), which were in large excess. Deuterated methoxy radicals, C D 3 0 , were also generated via the reaction of CD3 with N o t . Photoionization of the radicals was achieved using high intensity, dispersed synchrotron radiation, and ionization energies (IE) of these radicals were derived from the thresholds of the PIE spectra: IE(CD20H) = 7.54 f 0.02 eV, IE(CD20D) = 7.53 f 0.02 eV, and IE(CD3O) = 10.74 f 0.02 eV. The PIE spectra for CDzOH and CD30 are compared to those of a previous photoionization study, and differences are discussed. Integration of previously published photoelectron spectroscopy data for CDzOH yields a curve quite similar to our PIE spectrum. Empirical estimates of IE(CH20H) and IE(CH3O) are given to corroborate our assignments. Attempts to detect CH3O+ from direct ionization of CH30, which was generated by two methods (CH3OD F and CH3 NOz), were unsuccessful. However, HCO+, presumably formed along with H2 from C H 3 0 dissociative ionization, was detected at a threshold 8.73 eV. This appearance energy corresponds to a barrier of 17.6 kJ mol-' for the process CH@* HCO+ + H2. Heats of formation for neutral and ion species were determined using literature values for the proton affinity of CH2O and integrated heat capacities and the I E s measured in this laboratory, and an energy level diagram was developed for T = 298 K. For hydroxymethyl and methoxy radicals the following values were obtained: AfH02g8(CH20H) = -20.4 kJ mol-' (which agrees well with the recent results of Traeger and Holmes, J . Phys. Chem. 1993, 97, 3453); AfHo2ss(CH30) = 12.2 kJ mol-'. Bond dissociation energies were also computed: DOo(H-CH20H) = 393.1 kJ mol-' and DOo(CH3OH ) = 426.6 kJ mol-'. The measured ionization energies and the derived thermodynamic quantities are compared with previously reported results.

-

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+

+

Introduction Hydroxymethyl and methoxy radicals are important intermediates in both combustion' and atmospheric2 chemistry while CH20H+ plays a role in interstellar proce~ses.~ Accurate values of AfHOo(CH20H) and AfHOo(CH30)areessential in establishing bond energies in methanol: Dao(H-CH20H) and DOo(CH30H). However, even though both species have been intensely studied,"lO the thermochemistry of neutral CH2OH and C H 3 0 are still debated."-13 Among theoretical calculated values of AfHO for the title radicals and their ions have converged only recently.23-26 In recent studies, the ionization energies (IE) for CH20H,27a928a CH20D,Z7a and CD20H28b were measured directly by using photoionization mass spectrometry (PIMS). Thevalues reported from these PIMS studies for IE (7.56Z7a and 7.54928a eV for CHIOH, 7.W78 eV for CHzOD, and 7.54028 eV for CDzOH) agreed with and thus confirmed those obtained earlier by Dyke and ~o-workers,~9.30 who used photoelectron spectroscopy (PES). Additionally, the recent calculations of Curtiss et al.,23 performed at the G2 level of theory, give IE(CH20H) = 7.45 eV, which is

* Author to whom correspondence should be addressed.

Also at Department of Natural Scienccs, Coppin State College, Baltimore, MD. *Abstract published in Aduance ACS Abstracts, March 15, 1994. f

0022-3654/94/2098-4026%04.50/0

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in reasonable agreement with the experimental value of 7.56 eV that was determined both by Dyke and co-workers29Jo and by Tao et al.27aand with the value of 7.549 eV reported by Ruscic and Berkowitz in a "preliminary" experimental study.28a Ruscic and Berkowitz28balso derived a value of IE(CH20H) = 7.549 eV from their measurement of IE(CD2OH). (It is puzzling that the wavelength threshold for the photoionization of CH2OH quoted in ref 28a, 1641.5 A, does not agree with their energy value, 7.549 eV, but instead gives an energy value of 7.553 eV.) In the case of CH30, however, the situation has been less clear. The value for IE(CH3O) reported by Dyke30 (7.37 eV) differed substantially from that derived by indirect methods (about 10.7 eV,31-34as noted p r e v i o ~ s l y ~ ~Other ~ , ~ ~experimental ). values for IE(CH30) have been derived from an electron-impact, mass spectrometry study (8.5-8.8 eV)35 and a charge-inversion investigation (8.3 eV).36 A direct PIMS determination of the IE for C D 3 0 (10.726 eV) was reported recently by Ruscic and Berkowitz28 which agrees well with the calculatedvalueof Curtiss et al.,23IE(CH30) = 10.78 eV. In the present study, we report photoion efficiency (PIE) spectra from measurements on hydroxymethyl (CD20H and CD2OD) and methoxy (CH30 and CD30) radicals. The neutral radicals were generated in a discharge flow tube and sampled via a molecular beam nozzle. Direct, single-photon ionization was 0 1994 American Chemical Society

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4021

Photoionization of Hydroxymethyl and Methoxy accomplished by using dispersed synchrotron radiation with subsequent detection of mass-selected ions.

Experimental Section Experiments were performed by employing a discharge flowphotoionization mass spectrometer (DF-PIMS) apparatus coupled to the U-1 1 beamline at the National Synchrotron Light Source (NSLS).27aS7q38 The flow tube employed in this study was similar in design and operation to one used for kinetic measurements.27b The molecular beam source was a free-jet operating in the transition region. It was therefore necessary to determine the molecular beam density at the ionizer (3.0 cm from the nozzle) experimentally, and this was accomplished by comparing the NO ion signal level from a NO/He beam with the NO ion signal from N O as background gas in the detection chamber. In this way, we estimate the density of the molecular beam, at the ionizer, to be 180-200 times smaller than that in the flow tube. Hydroxymethyl and methoxy radicals were generated by the reaction of F atoms with methanol (CDSOH,CD30D), e.g.:3943

F + CD30H

-

CD,OH

+ DF

(la)

+

CD30 HF

(1b)

Fluorine atoms were produced a t the upstream end of the flow tube in a microwave discharge of FZ in helium carrier gas. Methanol was introduced into the flow tube through a movable injector, the tip of which was typically located 2 cm from the nozzle. The methanol concentration in the flow tube (approximately 5 x 10'3 molecules 6111-3) was usually about 10 times the [F]. The loss of radicals on the wall was minimized by using either a Teflon coating on the Pyrex tube27-37or a Teflon tube insert. The experiments were conducted a t ambient temperature (298 f 2 K),with flow velocities of 800-1000 cm s-' and flow tube pressures of 2-4 Torr. In addition to formation via reaction lb, methoxy radicals (CH3O/CD3O) were produced by the reaction of CHJCD3 with NO2 in the reaction sequence

F + CH,/CD, CH,/CD,

-

+ NO,

CH3/CD3+ HF/DF

-

CH,O/CD,O

+ NO

(2)

(3)

The general conditions were similar to those given above except that, in this case, methane (CHJCD4 at about 8% v/v in He) was added through the movable injector along with NO2 (2-3% v/v in He). With a value for k2 of 4-6 X 10-l' cm3 molecule-' s-1,39943 and with [CD,] = 5 X lot4molecule ~ m -the ~ , first-order rate for reaction 2 was 2-3 X 104 s-1. Thus, reaction 2 could be 99% complete within 1 cm from the injector tip (assuming instantaneous mixing). The rate constant for CD3 NO2 is also large, presumably similar to that for CH3 + NOz:" k3 = 2.5 X 10-1' cm3 molecule-' s-1. With [NO21 = 2.5 X 10'3 molecule cm-3, reaction 3 was therefore -85% complete within 2 cm from the injector tip and thus the injector was set, in these experiments, with the tip at that distance from the nozzle. The subsequent reaction of CHsO/CD30 with N02,45,46which could remove some methoxy,

+

CH30/CD30+ NO,

-

CH,ONO,/CD,ONO,

-

CH,O/CD,O

(4a)

+ HONO/DONO (4b)

is about 10 times slower than reaction 3 under the present conditions; operating at short reaction times, about 2-3 ms, would have caused the loss of methoxy via reaction 4 to be relatively small. The gaseous mixture in the flow tube was sampled as a molecular beam that was formed in a free-jet expansion (1-mm

nozzle, 1.5" skimmer/collimator). The source chamber and detection chamber were maintained at about 1 X 10-4 and 5 X 1WTorr, respectively. Measurements of photoionization spectra were carried out using tunable vacuum-ultraviolet (VUV) radiation a t the NSLS. A monochromator with a normal incidence grating (1200 lines/") was used to disperse the VUV light, and a LiF filter (A 1 103 nm) was used to eliminate secondand higher-order radiation. The nominal monochromator slit width was 750pm, and the resulting spectral bandwidth (fwhm) was 0.23 nm.38a At a nominal ring current of 500 mA, the light intensity in the ionizing region of the mass spectrometer was typically 10'3 photons per second a t the 0.23 nm bandwidth, as measured by using a calibrated ph0todiode.3~~The zero-order setting for calibration of the monochromator was adjusted to fO.O1 nm a t the beginning and checked at the end of each filling of the VUV ring. Typically, variations between the beginning and ending settings of zero-order corresponded to variations in the wavelength calibration of about f0.05 nm. Previous studies from this laboratory provide IE measurements that display good agreement (fO.01 eV, see respective references) with accepted literature values, e.g., IE(CH3OH) = (10.85 f 0.03) eV,27aIE(CH3) = (9.85 f 0.03) eV,37a and IE(Br2) = (10.51 f 0.02) eV.37b This close agreement therefore supports the accuracy of our wavelength calibration. Ions were mass selected and detected by using a quadrupole mass spectrometer (Extrel Model C50, operated in ion counting mode with 2.75 kV on the channeltron detector) that was aligned axially with the molecular beam. To obtain a photoion efficiency spectrum (or curve), ion intensity was measured relative to light intensity as a function of wavelength. The ion intensity was measured as ion counts (with the mass spectrometer in ion counting mode, a fast pulse pre-amplifier (MIT Model F-100T), and associated pulse counting electronics). The light intensity was monitored as a relative quantity by using a sodium salicylate scintillator and an attached photomultiplier (PM). This scintillator has a constant quantum yield in the VUV region; therefore the PM output was directly proportional to the absolute light intensity. The P M signal was processed first with a picoammeter and then with a voltage-tofrequency converter to ultimately obtain the light intensity in digital form. The ion counts and the light counts were accumulated simultaneously, at each wavelength step, in a digital data acquisition system for a prescribed integration time. All photoion efficiency spectra were obtained this way by plotting the ratio ion counts/light counts as a function of wavelength. Typical integration times were 30 s per point for survey scans (e.g. Figure la) and 60 s per point for threshold scans (e.g. Figures 2 and 3). Methanol samples, CH3OD (Fluka, puriss. grade, >99.9% D), CDjOH (MSD Isotopes, 99.8% D), and CD3OD (Fluka, puriss. grade >99.95% D), were thoroughly outgassed before use. In addition, each sample was carefully analyzed by low-energy (15 V, nominal) electron impact ionization/mass spectrometry and by PIMS. Isotopic impurity levels were determined from peak heights in mass spectra, and it was assumed that there were no isotope effects on instrumental sensitivity for detection of isotopomers. The results obtained from analyses with the two methods are summarized in Table 1 in terms of concentration of impurity relative to that of the corresponding sample molecule. Corrections pertaining to these impurities are discussed later. Methanol vapor was diluted with helium to about 0.05 mole fraction, and the mixtures were stored in 2-L bulbs. Methane (MG Industries, Scientific grade, 99.9995%) and m e t h a n e 4 (MSD Isotopes, 99.6% D) were used directly from cylinders. Nitrogen dioxide (MG Industries, CP grade, 97%) was purified by allowing the sample to stand with 0 2 followed by outgassing a t 77 K and bulb-to-bulb distillation. Purified NO2 and N02/ H e mixtures were kept in blackened bulbs to avoid photolysis.

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994

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4

TABLE 1: Analyses of Isotopic Impurities in Methanol Samples’ samDle imDurity m/z limpurityl/[samplel CHoOD CHsOH 32 4.6 X ( m / z = 33) CH2DOH 33 4.3 x 10-3 CH2DOD 34 1.6 x 10-3 CD3OH CD2HOHb 34 3.9 x 10-3 ( m / z = 35) CD2HOD 35 9.2 X l e CD3OD 36 1.9 X 1V2 CD,OD CD30Hand 35 3.5 x 10-2 ( m / z = 36) CD2HOD 4 Analyses with PIMS (Brookhaven National Laboratory) measured mass ratios of parent molecule peaks while analyses with low-energy electron impact ionization (NASA/Goddard Space Flight Center) measured fragmentation patterns. The natural abundance of I3C was taken as 1.1 1 X 10-2,e.g., 1.11% 13CH30Din the CH3OD sample. This isotopomerwas the only detectable impurity, at m / z = 34, in the CD3OH sample.

0.15

Ly

0

0.10

z

0

0.05

h 0.00

8.00

9 z

6.00

0

4.00 L I

2.00 0.00 110 ~

120

140 150 WAVELENGTH (nm)

130

160

170

Figure 1. Photoion efficiencyvs wavelength at m / z = 33 (CDzOH), 110 n m l X 5 170nm: (a) thisstudy;(b)ref28b(photoionyieldvswavelength). 30.006

I

I

. i 3

a

0.005

Lr

Y

-g

z” 0

z a

0.002 0.001

Kuo et al.

\

that reported by Ruscic and Berkowitz,28b the general features of the present result differ in several ways from theirs, which is plotted in Figure 1b for comparison. First, in the threshold region in Figure l a (at the first steplike feature), the photoion efficiency is about l/loo that of the maximum PIE at about 120 nm. In contrast, the photoion yield a t threshold in Figure 1b is about I / ~ o thatofthemaximumvalueat 150nm. ThesmallPIEat threshold observed in the present study was expected because, as discussed by Dyke et al.,29 the C-O bond length in CH2OH becomes shortened upon ionization and the carbon geometry changes from pyramidal to planar (leading to small Frank-Condon factors at threshold). Indeed, we have performed an integration of Dyke’s PES spectrum for CD20H (Figure 14of ref 30), and the resulting curve (146-166 nm or 8.5-7.5 eV) matches our PIMS result (Figure la) reasonably well but shows poor agreement with Figure lb. Second, the spectrum shown in Figure l a differs from that in Figure 1b in general shape and in location of the maxima. Our spectrum shows a very broad maximum centered at approximately 120 nm (almost a plateau a t X I120 nm) while Ruscic and Berkowitz28b reported a maximum photoion yield at about 148 nm, a plateau from 144 to 134 nm, and then a broad “valley” between 135 and 108 nm. Those authors did not comment on the decrease in their photoion yield spectrum that commences around 135 nm, and we can offer no explanation either. We do note, however, that the onset of increasing photoion yield in Figure 1bat about 114 nm occurs near the ionization threshold of possible methanol isotopic impurities (e.g. CHzDOH or CH30D) and *3CD20. Finally, Ruscic and Berkowitz28b observed evidence of a hot band in their spectrum at about 7.4 eV (1 67.5 nm); however, there is no indication of such an effect in the present work. Although reasons for these differences in the CD2OH spectra are not readily apparent, we may mention three dissimilarities between the experimental apparatus and procedures. First, Ruscic and Berkowitz28employed a many-line H2 lamp while the present study was performed using the high-intensity and continuum source of the U-11 beamline of the NSLS (which is 100-1000 times more intense than the laboratory light source). Second, the radical generator employed by Ruscic and BerkowitzZ8is an effusive source (with a 2-3 cm long reactor cup) whereas the present work was performed using a free-jet, molecular beam source (with a conventional 1 m long flow tube reactor). Both the extent of thermalization in the reactor and the beam density, therefore, were greater in the present work than in the previous studies.28 Finally, we employed a scintillator/photomultiplier tube light detector while Ruscic and BerkowitzZBaused a bare photomultiplier. The sodium salicylate scintillator/photomultiplier tube detector has a flat response vs wavelength while a bare photomultiplier tube should have a quantum yield curve characteristic of the photocathode. In Figures 2 and 3, photoion efficiency spectra at threshold are shown for CD2OH and CD20D, respectively. Both spectra have

: i

162 0.000

163

164 - -

- 165 --

-

- 166

167

WAVELENGTH (nm)

Figure 2. Photoion efficiency near threshold for m / z = 33 (CD2OH). The threshold at 164.5 nm gives an ionization energy of 7.54 eV.

Figure 3. Photoion efficiency near threshold for m / z = 34 (CD20D). The threshold at 164.6 nm gives an ionization energy of 7.53 eV.

Helium (MG Industries, 99.9999%)and F2 (MG Industries, 99.9% purity, diluted in helium at 2% v/v) were used as supplied. Results and Discussion

Photoion efficiency (PIE) spectra for CDzOH and CDzOD are shown in Figures 1-3. In Figure la, 110 IX I 170 nm, the CD2OH spectrum shows weak steplike behavior in the region just above threshold (at 164.4 nm) and then more or less monotonically increasing photoion yield with decreasing wavelength. Although the threshold observed in this workagrees with

Photoionization of Hydroxymethyl and Methoxy

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4029

~0.016

20.012

a

2 0.012

I

/

.eE

I

"AM

$

Y

Y

E

O'Oo8

E U E: cr w 0.004

z 0

E

y r - - r

0

E

a 0.000

! I

I , ,

110

,, ,,

I , I

I I , ,

, , ,,-,

0.000

113 116 WAVELENGTH (nm)

119

Figure 4. Upper trace ( 0 ) :CDaO photoion efficiency vs wavelength, uncorrected for methanol impurity (CDzHOH), microwave discharge on, CD3OH + F CD3O + HF. Lower trace (0):methanol impurity (CDzHOH), m / z = 34, microwave discharge off.

-

signal-to-noise ratios of about 20:l and appear to display linear behavior at threshold (see Guyon and B e r k ~ w i t zand ~ ~ Rosenstock48 for detailed discussions of experimental photoionization threshold behavior). Ionization energies were derived from extrapolations of the linearly ascending portions of the spectra down to background levels. Since the linear thresholds span 1.01.2 nm (about 5 times the spectral resolution of 0.23 nm), the slit function should not significantly perturb the determinations of the photoionization 0nset.4~The I E for CD2OH (7.54 f 0.02 eV) is in excellent agreement with those reported by Ruscic and Berkowitz28b (7.540 eV, using PIMS) and by Dyke30 (7.55 eV, using PES). The IE for CD2OD (7.53 f 0.02 eV) is slightly smaller than that reported by Dyke30 (7.56 eV, using PES), but thevalues agree within thecombined uncertainties. In both cases (CD20H and CD20D), theuncertainties (f0.02 eV) in the present IE values were conservatively estimated to be twice the spectral slit width, 0.23 nm. Although we employed a linear extrapolation in the analyses of these thresholds, we note that half-rise determinations would give 164.05 nm (7.56 eV) for CD20H and 164.15 nm (7.55 eV) for CD20D. Also, we note that careful examination of threshold regions (in Figure 2 and in Figure 5 as well) reveals (1) a systematic shift between our data and those of Ruscic and Berkowitz,ZBb with theirs being 0.10-0.15 nm higher than ours, and (2) the density of data points in our threshold determinations is quite similar to theirs.28b As mentioned in the Experimental Section, C D 3 0 was generated via two different methods. The PIE spectrum (1 10119 nm) for C D 3 0 produced via F CD3OH is shown in Figure 4. The sharp threshold at 115.4 nm is a striking feature. Also shown in Figure 4 is the PIE spectrum for the CD2HOH impurity, at m / z = 34, in the CD30H sample. After correction for the ion signal due to the impurity, the structure in the PIE spectrum above threshold was essentially removed. This is clearly seen in Figure 5, which also shows the PIE spectrum for C D 3 0 which was produced via reaction 3: CD3 N02. In both cases, the photoionization threshold occurs a t 115.4 nm and thus IE(CD30) = (10.74 f 0.02) eV. This value is in good agreement with that reported by Ruscic and Berkowitz,28 10.726 f 0.008 eV. These experimental values for IE(CD30) are somewhat smaller than the value for IE(CH30) calculated by Curtiss et al.,23 10.78 eV. Finally, the structure above threshold observed by Ruscic and Berkowitz28b for CD30, supposedly related to vibrational levels in the ion, is absent in the PIE spectra obtained in this study. Since the PIE spectra shown in Figure 5 were obtained by using two different C D 3 0 precursors and since they are in such good agreement, the present results suggest that the vibrational structure in the photoion yield spectrum reported by Ruscic and Berkowitz28b may have been due to an artifact.

+

+

,

T

I

I I I I

/ / / ,I 1 I

110

/ / I I / 8 I / / /

I I

113 116 WAVELENGTH (nm)

119

Figure 5. CD3O photoion efficiencyvs wavelength by two methods.Upper spectrum (0): CD3OH + F + CD3O + HF, corrected for methanol impurity (CDzHOH), m / z = 34. Lower spectrum (0):CD3 + NO1 CD3O + NO. Dashed line indicates the threshold at 115.4 nm (10.74 eV).

-

Direct photoionization of CH3O has apparently not been previously reported. Burgers and H0lmes3~estimate a lifetime for CH30+of IE(R0H) > IE(RNH2). This trend is observed as well in the ionization energies of the vinyl species, IE(CHz=CHX): X = F, 10.36 eV; OH, 9.14 eV; NH2, 8.20 eV. Therefore, for hydroxymethyl radicals we may expect that IE(CH20H) is bounded by IE(CH2F) = 9.05 eV and IE(CH2NH2) = 6.1 eV, Le., IE(CH20H) = 7.6 f 1.5 eV. Interestingly, the difference quantities, IE(CHz=CHX) - IE(CHzX), are fairly close for X = F and NH2, namely 1.3 and 2.1 eV. This suggests that IE(CH2OH) should be some 1.7 f 0.4 eV less than IE(CH2=CHOH), and thus we estimate that IE(CHzOH) = 7.4 f 0.4 eV. The two estimates for IE(CH20H), 7.6 f 1.5 eV and 7.4 f 0.4 eV, agree very well with the experimental value of about 7.5 eV obtained in PES studies29J0 and in the present and p r e v i o u ~ 2 7 PIMS ~3~~~ studies. This level of agreement suggests that it may be worthwhile to pursue estimation procedures for IE(CH3O) since the reported studies display such disparate results: 7.37 eV for CHBOvia PES30 and 10.726 and 10.74 eV for C D 3 0 from previousZ8and present PIMS studies, respectively. However, in the case of CHjO, it is not possible to estimate the ionizattion energy by the same procedure as with IE(CH20H). Nevertheless, there are two other related methods that could be useful. First, it is now a folklore of ion energetics that increasing the size of the alkyl group, R, adjacent to the site of ionization, X, in a species R X

-

(I)

or, rearranging, IE(CH3-0)

- -

- IE(CH3-OH)

(11) Equation I1 yields a value of 1 1.24 eV which can replace I E ( H 0 ) = 13.00 eV as an upper bound to IE(CH3O). Thus, the ionization energy of CH30should be between 9.2 and 1 1.24 eV. Pursuing this line further, wecan provide bounds for the differencequantity in eq I between the corresponding quantities for NH2 and F, Le., IE(H-NHz) - IE(CH3-NH2) < IE(H-0) - IE(CH3-0) < IE(H-F) - IE(CH3-F). The upper and lower bounds computed in this way are 9.43 eV I IE(CH30) I1 1.8 1 eV. A simple average of the four estimated boundingvalues (9.2-1 1.24and 9.43-1 1.8 1) yields IE(CH30) = 10.42 f 1.30 eV. The present experimental results for IE(CD20H), IE(CD2OD), and IE(CD30) and empirical estimates for IE(CH2OH) and IE(CH3O) are compared in Table 2 with previously reported values. Our ionization energies for hydroxymethyl are generally quite consistent with those reported previously, although the trend in decreasing IE with increasing extent of deuteration, observed in the results from this laboratory, is not corroborated by the PES study.30 On the other hand, the combined results of the empirical estimates for IE(CH30),notwithstanding the approximate nature of these estimation methods, clearly favors the "high" experimental value of 10.73-10.74 eV (for CD30) obtained by using PIMS in the present work and in the work reported by Ruscic and Berkowitz.28 The value calculated by Curtiss et aLz3at the G2 level of theory, 10.78 eV, also corroborates the "high" value. We conclude, therefore, that the earlier "low" values for IE(CH30) reported by Dyke,30 Hoyermann et a1.,35 and Griffiths and Harris36 were in error. An energy level diagram, shown in Figure 7, for the CH2O H / C H 3 0 system was developed using the ionization energies measured in this laboratory along with other values from the literature. This diagram is similar to one reported by Burgers and Holmes,34but it includes some more recent and more precisely determined quantities. The starting point in the present study was the determination of ArH0298(CH20H+). First, from the proton affinity of CH20,711.8 kJ mol-' calculated by Smith and Radom at the G2 level of theory,26 AfHo2,,(CH20H+) = AfHoz,8(CH,0)

+ AfHo298(H+) PA(CH20) (111)

wederiveavalueof AfH0298(CHzOH+)= 709.5 kJmol-I. Second, from the enthalpy change for the proton-exchange reaction CH2O + HCNH+ CH20H+ + H C N (ArHO298 = 0.42kJ we derive a value AfHo298(CH20H+) = 706.3 kJ mol-'. The averageof the twovalues is AfH029s(CHzOH+)= 707.9 kJ mol-'. Next we compute the enthalpy of ionization for CHzOH at 298 K:

-

AjYI(CH2OH)

W C H z O H ) + (H0Z98 - H'O)~HZOH+ (H0298 - H0&CH20H (Iv)

Values for the integrated heat capacities were taken from Ma et

Photoionization of Hydroxymethyl and Methoxy

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4031

TABLE 2 Comparison of Ionization Energies for Hydmxymethyl and Methoxy Radicals precursop

hydroxymethyl radical

CH3OH CH3OD CDpOH CDoOD

CH20H CH2OD CD20H CD20D

I E (eV) 7.56,29J07.56:" 7.549,28' 7.45:3 7.55,"o 7.597' 7.55,'" 7.540,28.-28b7 . W 7.56,'O 7.53c

methoxy radical

IE (eV)

CH3O CD3O

7.37,"' 8.5-8.8,"' 8.30,"6,d (10.7); 10.78,2410.426 10.726,28a+28b 10.74cJ

7Sb

a Precursor indicates the methanol isotopomer that was used to generate the indicated hydroxymethyl and methoxy radicals (Le., on the same line) via reaction with F atoms. Exceptions are noted in other footnotes. Values determined in this laboratory are in bold print. 6 Present work, empirical estimate. Present work, experimental result. Derived from translational energy-loss measurements of CHpO-generated via a charge-inversion reaction of CHpO+ with Xe atoms. The CH3O+ ions were generated in an electron impact/chemical ionization source. Determined indirectly from estimates of AfHO(CHIO*); see text and refs 27a and 31-34.lIdentical results were obtained from direct ionization of CD3O that was generated via CD3OH + F and CD3 + N02.

explanation for the difference between their work and that of Seetula and Gutman." However, a discrepancy of 4-5 kJ mol-' remains. Additionally, the present value, -20.4 kJ mol-l, is slightly more negative than that derived by Ruscic and Berkowitz,28b -1 5.5 kJ mol-', and that calculated by Espinosa-Garcia and Olivares del Valle,25-15.6 kJ mol-'. The heat of formation of CH2OH at 0 K is computed from

\

\ \

707.9

-

+

AfHoo(CH20H)= AfHo2g8(CH20H)

',i CH20H*

(HoO

2 \

--L/;2.2

\

-20.4-

CH,OH

Figure 7. Energy level diagram for the CH2OH/CH,O system; energies are in units of kJ mol-'. See text for discussion and references. The lowest Rydberg state of CH3O was estimated to be 0.75IE; see ref 27a.

a1.,59(H02g8 - H'O)CH~OH+ = 10.21 kJ mol-', and from Traeger and Holmes,'2 (H0298 - Hoo)CH20H = 11.31 kJ mol-'. The value IE(CH20H) = 7.56 eV was selected because of the excellent agreement between our earlier PIMS study27aand the PES study of Dyke et al.29 The result, M I ( C H ~ O H = ) 728.3 kJ mol-', was then used to derive AfH0298(CH20H):

- Ho298)CH20H - x ( H o O

- H0298)elemcnts ("1

Integrated heat capacities were taken from Traeger and Holmes,'Z (H'o - H0298)CH20H = -1 1.31 kJ mol-', and from the JANAF tables,6O ( H O o - H0298)c = -1.051 kJ mol-l, 1.5(Hoo- H'298)~~ = -12.701 kJ mol-', and 0.5(H00 - p298)02 = -4.342 kJ mol-'. The value thus computed was AfHOo(CH20H) = -1 3.6 kJ mol-'. The heat of formation at 0 K of the hydroxymethyl ion is then computed from AfHOO(CH20H) and IE(CH2OH) to be 715.8 kJ mol-'. Next, we calculate AfHOo(CH30) from AfH'o(CH20H) and the energy difference, AE,between CH2OH and C H 3 0 at 0 K. Recently computed values for AE are 29.3-37.7 kJ mol-' (Bauschlicher et aLZ4)and 36.8 kJ mol-' (Curtiss et al.23) while an evaluation by Batt e f al.8 derived 31.4 kJ mol-'. From these, we select a value for AE of 33.5 kJ mol-' with an estimated uncertainty of about 4 kJ mol-' (D6b6 et al.13 recently concluded that AE = 34.0 kJ mol-'). Taking this estimate of the energy difference, AfWo(CH30) = 19.9 kJ mol-'. The heat of formation at 0 K of the methoxy ion is then computed from AfHoo(CH30) and IE(CH30). Avalueof IE(CH3O) = 10.75 eV was obtained by correcting the value for IE(CD30) = 10.74 eV by 0.01 eV to account for the maximum possible isotope effect;23 and the result is AfHoo(CH30+) = 1057.1 kJ mol-'. The heat of formation at 298 K of the methoxy radical is computed according to

AfHo2,,(CH20H) = AfHo298(CH20H+)- AHI(CH20H)

= -20.4 kJ mol-'

(V)

This value for the enthalpy of formation of the CH2OH radical is in reasonably good agreement with the value of -18.9 kJ mol-' recently derived by Traeger and Holmes,12 who started from an appearance energy measurement. The present result therefore corroborates the "low" value for AfHo298(CH20H)andchallenges the "high" values of -8.9 kJ mol-' reported by Seetula and Gutman" and -9 kJ mol-' reported by D6bt et al.,'3 both of whom used second-law and third-law calculations to derive their results. We note further that Traeger and Holmes employed IE(CH20H) = 7.55 eV, whichwasderived from themeasurement of IE(CD2OH) = 7.54 eV reported by Ruscic and Berkowitz2*b If the derivation of Traeger and HolmesIz is re-evaluated by employing IE(CH20H) = 7.56 eV, the value AfH0298(CH20H) = -19.8 kJ mol-' is obtained, that further widens the gap between their evaluation12 and that of Seetula and Gutman." It should also be mentioned that Traeger and Holmes12 offered a partial

Integrated heat capacities of the elements were taken from the JANAF tables,6O and ( H O o - H0298)CH30 = -10.40 kJ mol-' was taken from Wong and RadomS6' The result is AfHo2ss(CH30) = 12.2 kJ mol-l. Finally, we compute the enthalpy of ionization at 298 K for CH30, MI,and the heat of formation at 298 K for CH30+:

The value for (HO298 -H0&~30+, 11.43 kJ mol-', was estimated from the integrated heat capacities for CH3OH+ (1 2.73 kJ mol-') and CH20+ (10.13 kJ mol-'), taken from Ma et al.,59 and IE(CH30) = 10.75 eV, as discussed above. Thus, M 1 ( C H 3 0 ) = 1038.2 kJ mol-', and then

4032

Kuo et al.

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994

AfHo2g,(CH30+)= AHI + AfH0298(CH30) (1x1

= 1050.4 kJ mol-' The remainder of the energy diagram (Figure 7) has been discussed previo~sly.2~aBriefly, the barrier to isomerization of the neutrals (CH20H CH30) has been calculated'9.23 to be at 125-150 kJ mol-' while the location of the barrier to isomerization of the ions (CH3O+ s CH20H+) is taken as the energy level (1 192 kJ mol-') of the excited singlet state of the methoxy i0n.I7J7a The barrier to dissociative ionization of CH2OH (+ HCO+ H2) was estimated previously27a,62"5 at about 920 kJ mol-' while the energy at which dissociative ionization of C H 3 0 (- HCO+ H2) occurs is 854.5 kJ mol-', as determined in the present study from the onset of HCO+. Finally, we may compute the C-H and 0-H bond dissociation energies66 for CH3OH by using the values derived above for ArHOo(CH20H) and AfHOo(CH30)along with heatsof formation atOK for theH-atom(216.04 kJmol-')49andmethanol(-190.70 kJ m01-I):~~

*

+

+

Doo(H-CH20H) = AfHOo(CH2OH)

+ AfHoo(H) AfH00(CH30H) (x)

= 393.1 kJ mol-' (94.0 kcal mol-') DOO(CH3O-H) = AfHOo(CH30)

+ A&Ioo(H)

-

AfHoo(CH30H) (XI)

= 426.6 kJ mol-' (102.0 kcal mol-') These dissociation energies are in relatively good agreement with values reported by Golden and Bensod (corrected to 0 K) or derived from the evaluation of Lias et ~ 1 . (C-H, ~ 9 387.4 kJ mol-' and 0-H, 430.1 kJ mol-'). Reasonable agreement also exists between the present results for Doo(C-H and 0-H) and those of Ruscic and Berkowitz28b(C-H, 396.2 kJ mol-' and 0-H, 43 1.4 kJ mol-l) as well as that of Espinosa-Garcia and Olivares del Valle25 (C-H, 396.1 kJ mol-'). Comparison with values derived from recent high-level calculations23~24shows poorer agreement (C-H, 402.5 kJ mol-' or 96.2 kcal mol-' and 0-H, 439.3 kJ mol-' or 105.0 kcal mol-'). However, by using ArHOo(CH20H) and AfHoo(CH30) from Curtiss et al.23 and other values from Lias et ~ 1 . (as 4 ~ in eqs X and XI), we calculated D0o(H-CH20H) = 397.5 kJ mol-' and DOo(CH30-H) = 434.3 kJmol-I, which differ by 5.0 kJ m0l-'36~from those of Curtiss et al.23 but are in closer agreement with our results and with those derived from Lias et Also, thevalues reported by Bauschlicher et aLZ4 werescaled from their calculated ones by +8.4 kJ mol-' (2 kcal mol-') for C-H and 16.7 kJ mol-' (4 kcal mol-l) for 0-H (which probably increases their uncertainties by 5-10 kJ mol-').

+

Conclusions In the present PIMS study, ionization energies for CD20H, CD20D, and CD30 were determined directly from photoion efficiency thresholds. The IE values for the hydroxymethyl radicals (7.54 and 7.53 eV, respectively) corroborate the PES results reported by Dyke.30 The IE for CD30 (10.74 eV) agrees well with a previous PIMS study28 and our empirical estimate of IE(CH30) = 10.4 f 1.3 eV. Additionally, therecent calculations of Curtiss et al.23 yield IE(CH3O) = 10.78 eV. Therefore these results clearly indicate errors in earlier s t ~ d i e s " J . ~that ~ J ~reported "low" values for IE(CH30). Despite thegood agreement between the present results for IE(CD20H) and IE(CD30) and those of Ruscic and Berkowitz,28we observed several differences in the structure of the corresponding photoion curves. In particular, we note that the present results for C D 3 0 indicate that the

vibrational structure above threshold reported by Ruscic and Berkowitz2Sb may have been due to an artifact. Also, an integration of Dyke's PES spectrum for CD2OH (Figure 14 of ref 30) yields a curve that matches our PIMS result reasonably well (146 nm IA I166 nm). In contrast, this integrated curve displays poor agreement with the PIMS result for CD20H reported by Ruscic and Berkowitz.28b Heats of formation for neutrals and ions were computed at 298 and 0 K for the CH20H/CHpO system to construct an energy level diagram. The value derived for AfHO298(CH20H) is -20.4 kJ mol-', which agrees with some earlier results and that reported recently by Traeger and Holmes.I2 This result is at odds, however, with those from two other recent studies (based on kinetics experiments) reported by Seetula and Gutman" and D6bC et al.13 Finally, bond dissociation energies for C-H and 0-H in methanol were computed from the AfHovalues derived in this investigation: DOo(H-CH20H) = 393.1 kJ mol-' (94.0 kcalmol-'),D0298(H-CH20H) = 399.3 kJmol-I (95.4kcalmol-l), DOo(CH30-H) = 426.6 kJ mol-' (102.0 kcal mol-'), and D0298(CH30-H) = 431.9 kJ mol-' (103.2 kcal mol-'). These results are in reasonably good agreement with values derived from earlier e~aluations;s~~9 however, they agree less well with results from recent high-level c a l c ~ l a t i o n s . ~The ~ ~ apparent ~~ discrepancy might be reduced by adjustment of scaling values that were applied to the theoretical results, and an apparent difference in ArHOo(H) and AfHoo(CH30H), between Curtiss et al.23 and Lias et ~ 1 . : ~ needs to be resolved. Acknowledgment. J.F.L. wishes to thank Dr. S.G. Lias for stimulating discussionsduring a year of sabbatical leave at NIST. Preliminary results of this work were presented at the Third International ConferenceonChemical Kinetics (July 12-16,1993, held at NIST, Gaithersburg, MD) as Poster Paper R43. The work at Goddard Space Flight Center was supported by theNASA Upper Atmosphere Research Program. The work at Brookhaven National Laboratory was supported by the Chemical Sciences Division, Office of Basic Energy Sciences, US.Department of Energy, under Contract DE-AC02-76CH00016. References and Notes (1) (a) Warnatz, J. In Combustion Chemistry; Gardner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984, and references therein. (b) Grotheer, H.-H.; Kclm, S.;Driver, H. S.T.; Hutchwn, R. J.; Lockett, R. D.; Robertson, G. N. Ber. Bunsen-Ges. Phys. Chem. 1992,96, 1360 and references therein. (c) Driver, H. S.T.; Hutchwn, R. J.; Lockett, R. D.; Robertson, G. N.; Grothcer, H.-H.; Kclm, S. fbid. 1376. (d) Hagele, J.; Lorenz, K.; Rhasa, D.; Zellner, R. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1023. (2) (a) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampon, R. F., Jr.; Kerr, J. A.; Troc, J. J. Phys. Chem. Ref.Duru 1989,18,881 and references therein. (b) Nesbitt, F. L.; Payne, W. A.; Stief, L. J. J . Phys. Chem. 1988, 92, 4030 and references therein. (3) Huntress, W. Personal communication. Cited in: Gottlieb, C. A.; Ball, J. A.; Gottlieb, E. W.; Dickinson, D. F. Arrophys. J. 1979, 227, 422. (4) Cruicbhank. F. R.: Benson. S . W. J . Phvs. Chem. 1969.. 73.. 733. ( 5 ) Golden, D. M.; Benson, S.W. Chem. Rh. 1969, 69, 125. (6) Tsang, W. fnr. J. Chem. Kincr. 1976,8, 173. (7) Engelking. P. C.; Ellison, G. B.; Lineberger, W. C. J . Chem. Phys. 1978,69, 1826. (8) Batt, L.; Burrows, J. P.; Robinson, G. N. Chem. Phys. Lctt. 1981, 8, 467 and references therein. (9) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493.

(IO) Holmes, J. L.; Lossing, F. P. Int. J. Muss Specrrom. Ion Processes 1984,58, 113. (11) Seetula, J. A.; Gutman, D.J. Phys. Chem. 1992, 96, 5401. (12) Traeger, J. C.; Holmes, J. L. J. Phys. Chem. 1993, 97, 3453. (13) D6M. S.;Otting, M.; Temps, F.; Wagner, H. Gg.; Ziemcr, H. Ber. Bunsen-Ges. Phys. Chem. 1993. 97, 877. (14) Haney, M. A.; Patel, J. C.; Hayes, E. F. J. Chem. Phys. 1970,53, 4105. (15) Dewar, M. J. S.;Rzepa, H. S.J . Am. Chem. SOC.1977,99, 7432. (16) Schleyer, P. v. R.; Jemmis, E. D.; Pople, J. A. J. Chem. Soc. Chem. Commun. 1978, 190. (17) Dill, J. D.; Fiwher, L. L.; McLafferty, F. W. J. Am. Chem. SOC. 1979, 101, 6531. (1 8) Bouma, W. J.; N o h , R. H.; Radom, L. Org. Muss Spectrom. 1982, 17, 315.

Photoionization of Hydroxymethyl and Methoxy (19) Saebo, S.; Radom, L.; Schaeffer, H. F. J. Chem. Phys. 1983,78,845. (20) Rayez, J. C.; Rayez, M. T.; Halvick, P.; Duguay, B.; Lesclaux, R.; Dannenberg, J. J . Chem. Phys. 1987, 116,203. (21) Francisco, J. S.; Williams, I. H. Int. J. Chem. Kinet. 1988, 20, 455. (22) Sana, M.; Leroy, G. J . Mol. Srruct. (Theochem) 1991, 226, 307. (23) Curtiss, L. A,; Kock, L. D.; Pople, J. A. J . Chem. Phys. 1991, 95, 4040. (24) Bauschlicher, C. W.; Langoff, S. R.; Walch, S. P. J. Chem. Phys. 1992, 96, 450. (25) Espinosa-Garcia, J.; Olivares del Valle, F. J. J. Phys. Chem. 1993, 97, 3377. (26) Smith, B. J.; Radom, L. J . Am. Chem. SOC.1993, 115, 4885. (27) (a) Tao, W.; Klemm, R. B.; Nesbitt, F. L.;Stief, L. J. J . Phys. Chem. 1992, 96, 104 and references therein. (b) Klemm, R. B.; Nesbitt, F. L.; Skolnick, E. G.; Lee, J. H.; Smalley, J. F. J . Phys. Chem. 1987, 91, 1574. (28) (a) Ruscic, B.; Berkowitz, J. Photoionization Mass Spectrometric Studies of the Combustion Intermediates CH20H and CH30. Prepr. Pap. (Am.Chem.Soc.,Div. FuelChem.) 1991,315,1571. (b) Ruscic,B.; Berkowitz, J. J . Chem. Phys. 1991, 95, 4033 and references therein. (29) Dyke, J. M.; Ellis, A. R.; Jonathan,N.; Keddar, N.; M0rris.A. Chem. Phys. Lett. 1984, 111, 207. (30) Dyke, J. M. J. Chem. Soc., Faraday Trans. 2 1987,83,69. (31) Lossing, F. P.; Holmes, J. L. J . Am. Chem. SOC.1984, 106, 6917. (32) Wodtke, A. M.; Hiutsa, E. J.; Lee, Y. T. J. Chem. Phys. 1986,84, 1044. (33) Ferguson, E. E.; Roncin, J.; Bonazzola, L. Int. Mass Spectrom. Ion Processes 1987, 79, 215. (34) Burgers, P. C.; Holmes, J. L. Org. Mass Spectrom. 1984, 19, 452. (35) Hoyermann, K.; Loftfield, N. S.; Sievent, R.; Wagner, H. Gg. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1980; p 831 and references therein. (36) Griffiths, W. J.; Harris, F. M. Chem. Phys. Lett. 1987, 142, 7. (37) (a) Nesbitt, F. L.; Marston, G.; Stief, L. J.; Wickramaaratchi, M. A,; Tao, W.; Klemm, R. B. J. Phys. Chem. 1991, 95, 7613 and references therein. (b) Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo, S. C.; Klemm, R. B. Chem. Phys. Lett. 1993, 211, 416 and references therein. (38) (a) Grover, J. R.; Walters, E. A,; Newman, J. K.; White, M. C. J . Am. Chem. SOC.1985, 107, 7329 and references therein. (b) Grover, J. R. Private communication. (39) Bogan, D. J.; Kaufman, M.; Hand, C. W.; Sanders, W. A.; Brauer, B. E. J . Phvs. Chem. 1990. 94. 8128 and references therein. (40) MfCaulley, J. A.;'KeiIy, N.; Golde, M. F.; Kaufman, F. J . Phys. Chem. 1989, 93, 1014. (41) Durant, J. L., Jr. J . Phys. Chem. 1991, 95, 10701 and references therein. (42) Glanser, W. A,; Koszykowski, M. L. J . Phys. Chem. 1991,95,10705. (43) Khatoon, T.; Hoyermann, K. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 669. (44) Yamada, F.; Slagle, I. R.; Gutman, D. Chem. Phys. Lett. 1981,83, 409. ..

(45) McCaulley, J. A.; Anderson, S. M.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Lett. 1985, 115, 180. (46) Frost, M. J.; Smith, I. W. M. J . Chem. SOC.Faraday Trans. 1990, 86, 1751. (47) Guyon, P. M.; Berkowitz, J. J . Chem. Phys. 1971, 54, 1814. (48) Rosenstock, H. M. Inr. J. Mass Spectrom. Ion Phys. 1976.20, 139. (49) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref.Data 1988, 17 (Suppl. No. 1).

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4033

-

(50) The reaction CHI + NO2 CH2O + HNO is about 124 kJ mol-I more exothermic than reaction 3. (51) Frost, M. J.; Smith, I. W. M. J . Chem. SOC.Faraday Trans. 1990, 86, 1757. (52) McCaulley, J. A.; Moyle, A. M.; Golde, M. F.; Anderson, S. M.; Kaufman, F. J . Chem. SOC.Faraday Trans. 1990,86, 4001. (53) The correction for W H 2 0 was made by applying the natural abundance of I3C (1.11%) and using data for I2CH20 (which displayed a sharp threshold in the m / z = 30 PIE spectrum at 113.8 nm) obtained under identical conditions as the C H 3 0 data. (54) The correction for CHDO was made by taking 0.006 times the 12CH20 data that were obtained under identical conditions as the C H 3 0 data (assuming that the CH2DOH and CH2DOD impurities contributed equally to the generation of CHDO; see Table 1). (55) Unless otherwise stated, all thermodynamic values used in this discussion are taken from Lias et 0 1 . ~ ~ (56) For example, see the review: Levitt, L. S.; Widing, H. F. Prog. Phys. Org. Chem. 1976, 12, 119 and references cited therein. (57) Tanaka, K.; Mackay, G. I.; Bohme, D. K. Can. J . Chem. 1978, 56, 193. (58) From the measurement of Tanaka et aLs7we compute AfH0298(CH2OHt) = 706.3 kJ mol-I from49ArH0298(CH20) = -108.78 kJ mol-', AfH0298(HCN) = 135.14 kJ mol-', and AfHo298(HCNHt) = 949.8 kJ mol-' (the last value is thoroughly discussed in ref 37a). The uncertainy in the derived value for AfH0298(CH20H+)may be estimated by performing a propogation of error analysis. The uncertainties in AH(reaction), AH(CHzO), Y ( H C N ) , and AH(HCNH+) are k2.1, k0.7, k0.5, and k7.7 kJ mol-], respectively (the latter quantity was carefully re-evaluated because the uncertainty in M ( H CNHt) had previously been overestimated"*). The overall uncertainty in ArH0298(CH20Ht) isthereforeestimatedtobeill.0 kJmol-1. Thisapproach yields a value for AfH0298 (CH20Ht) that agrees well with that derived from the PA(CH20) value calculated by Smith and RadomZ6and displays a similar uncertainty. Furthermore, this approach avoids the debate over PA(H2O) discussed by Traeger and Holmes.12 (59) Ma, N. L.; Smith, B. J.; Pople, J. A.; Radom, L. J . Am. Chem. SOC. 1991, 113, 7903. (60) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985,14 (Suppl. No. 1). (61) Wong, M. W.; Radom, L. J . Am. Chem. SOC.1993, 115, 1507. (62) The barrier to dissociation of CH20H+ to form HCO+ Hz was estimated in ref 27a to be the "activation energy" for this process minus the excess energy in the products as derived by Williams and Hvistendahl.63 Bowen and and Richard et ~ 1 . 6 5 (63) Williams, D. H.; Hvistendahl, G. J. J . Am. Chem. SOC.1974, 96, 6753. (64) Bowen, R. D.; Williams, D. H. J . Chem. SOC.Chem. Commun. 1977, 378. (65) Richard, G. J.; Cole, N. W.; Christie, J. R.; Derrick, P. J. J . Am. Chem. SOC.1978, 100, 2904. (66) We may also compute Do values at 298 K in a similar manner: Do298(H-CH20H) = 399.3 kJ mol-' (95.4 kcal mol-I) and D0298(CH@-H) = 431.9 kJ mol-' (103.2 kcal mol-I). (67) This discrepancy of 5.0 kJ mol-1 (1.2 kcal mol-]) is apparently due to differences in either AfHoo(H)or ArH0o(CH3OH)or both between Curtiss et 0 1 . ~and Lias et ~ 1 . ~ ~

+